Radio frequency (RF)—and microwave-based sensing techniques, including cavity perturbation methods and the like, are used in many applications ranging from laboratory and research instrumentation to process control systems and even on-vehicle sensors. In many applications, antennas or probes are used to transmit and/or receive radio frequency or microwave signals to conduct these measurements. Oftentimes, additional information is required to accurately interpret the measurement results, as external factors, such as temperature or moisture for example, may introduce additional variables that influence the measurements.
There is a further need in many applications for robust sensing systems, capable of withstanding high levels of vibrations, large temperature variations, harsh or corrosive environments, exposure to the elements, and similar demanding requirements. Many of these requirements can only be met through complex, cumbersome, and expensive systems, which suffer from the following limitations.
First, current sensing systems employing antennas or probes to transmit and receive radio frequency or microwave signals require additional sensors to measure multiple parameters. Temperature sensors, moisture sensors, pressure sensors, and the like are often required (in addition to the antenna or probe) to conduct these ancillary measurements, adding additional cost and complexity to the system.
Second, many conventional antennas and RF/microwave probes are not robust and are incapable of surviving extended operation with exposure to high temperatures, high vibration levels, thermal shock, corrosive or dirty environments, oxidizing or reducing conditions, and the like. Environmental exposure, rain, snow, and salt water for example, as well as exposure to chemicals and solvents, such as oils, fuels, acids, and similar chemicals, is also detrimental to many conventional RF/microwave probes and antennas.
Third, RF/microwave probes and antennas designed for operation in harsh environments are generally expensive, large and bulky, and ill-suited for use in low-cost measurement systems. The expense in these systems stems from both the requirement to utilize more exotic and higher priced materials, as well as the added cost and complexity involved in the manufacture and assembly of the antenna or probe and associated connectors.
Fourth, conventional antenna systems suffer from limitations of variability introduced by reflections, losses, and incorrect impedance matching. Many of these sources of variability are related to the antenna design itself and the manner in which the antenna is connected to the signal cable. For example, most antennas require additional components to connect the antenna to the signal cable, such as interconnects and ancillary components. Not only do these components exhibit some inherent variability, which may also vary over time (corrosion, loosening, moisture induction, etc.), but they are also subject to operator error, including misalignment, over-tightening, under-tightening, reliability and the like.
Fifth, radio frequency connectors suitable for use in the microwave range are expensive, particularly when high temperature operation is required. Robust connectors, such as Type N for example, are also bulky, making them ill-suited for applications where a small form-factor is required. In some cases, the size of these connectors may be of the same order as the actual component or sensor.
Sixth, the use of an interconnect on the end of an RF probe or antenna, such as a conventional RF connector (BNC, SMA, Type N, and many others) allows for the connection of any type of cable to the antenna with the same type of matching connector, or with a different type of connectors through the use of a suitable adapter. In applications where the measurement, or operation of the system, requires the use of a cable with specific characteristics (impedance, length, temperature rating, or similar performance specifications), additional sources of error may easily be introduced when the connecting cable is replaced with another cable not meeting the required specifications. Thus, measurement systems in which the antennas and RF probes contain an interconnect, may be susceptible to errors introduced by the installation of incorrect cables.
Therefore, it would be beneficial if there were an RF probe or antenna system that addressed the problems described above. Such a system would be advantageous in that inherent variability in the probe or antenna performance would be reduced, and sources of operator error eliminated, ultimately improving the performance of the overall measurement system with which the antennas or probes are used. In addition, probes or antennas containing multiple sensing elements, or the ability to monitor multiple parameters, an RF signal and temperature, for example, or an RF signal and pressure in another example or an RF signal and gas or liquid composition in yet another example, would greatly simplify the measurement system, by allowing one or more parameters to be monitored with the same probe.
The probes and antennas described herein may be used in a number of applications, ranging from cavities to transmission lines, and even in free space. One range of applications include systems which monitor changes in the dielectric properties of a material or a mixture of materials in order to deduce some information regarding the state of the system. A particular example of such a system is a moisture measurement system used to monitor moisture levels in various materials. Another example includes fluid blend sensors, where the blend may be composed of one or more liquid, gas, or solid materials, and where changes in the dielectric properties of the mixture of materials may provide some information on the state of the system, such as the composition of the mixture, flow rate, or other parameters of interest.
Yet another example includes a class of radio-frequency measurement systems applied to monitor exhaust emissions or the state of various emission control devices. Radio-frequency or microwave systems used to monitor the loading state of particulate filters, such as the amount of soot or ash accumulated in a diesel particulate filter, is one exemplary application. Another application includes the monitoring of various gaseous species, such as oxygen or oxides of nitrogen (among others), adsorbed onto various catalytic emission aftertreatment components, such as three-way catalytic converters, selective catalytic reduction systems, oxidation catalysts, or lean NOx traps, to name a few. In yet another embodiment, the monitored parameter may be a change in the dielectric properties of the material itself, such as the filter material in the case of a diesel particulate filter, in one example and the catalysts substrate, washcoat, or catalyst material in yet another example. Although aftertreatment filters and catalysts are described and are particularly challenging, any filter system or catalyst system can use the technology described herein.
In many applications, there is a need for antennas or measurement probes suitable for extended operation over a range of conditions, including exposure to high temperatures, vibration, mechanical stresses, water and other liquids, and the like. In addition, such antennas or probes may require periodic removal for inspection, maintenance, or replacement. An antenna or probe system which is robust and exhibits little inherent variability is required to ensure proper operation of these systems, and also minimize errors introduced by operators when installing or replacing such antennas or probes.
Furthermore, many measurement systems suffer from variability introduced by changes in parameters other than the measurement parameter of interest. For example, external variables including moisture, flow, temperature, pressure, composition, or the introduction of contaminants, and the like may also affect the performance of radio-frequency or microwave measurement systems. A particular example is the determination of soot accumulation on particulate filters, in which case the dielectric properties of the soot also vary with temperature. Another example is the dielectric state of the catalyst material or the gas-phase species absorbed or adsorbed on a catalyst, or the liquid or solid phase species collected on a filter. Many other measurement systems exhibit similar cross-sensitivities to temperature and other parameters. It is, thus, desirable to monitor the various parameters that may affect the measurement using a single probe, as opposed to using one or more sensors, where each sensor only measures one specific parameter.
The antenna probe system described herein may be applied to any number of applications in which reducing the variability of the antenna is important to improve the signal. Other applications include those in which the number of additional sensing elements may be reduced through the use of a single antenna probe containing one or more sensing elements.